"... one of my colleagues ... " implies that this could be a split
(front-end/back-end) database, and that more than one person can be using
the application simultaneously.
If it just started showing this symptom, have you tried restoring the
database to an earlier point in time from a backup. This might help isolate
where the problem is. If the earlier version works, your "current" version
has the problem. If the earlier version also fails, something about your
network would be the next place to look.
Regards
Jeff Boyce
Microsoft Office/Access MVP